1. Field of the Invention
This invention relates to materials in which magnetic bubble domains can be generated and, more particularly, to a multilayered composite including a layer of magnetic bubble domain material suitable for the selective generation of magnetic bubble domains.
2. Brief Description of the Prior Art
It is well known in the art to use magnetic materials such as garnets and orthoferrites with intrinsic and/or induced (by shape, stress or growth) uniaxial anisotropy to generate magnetic single wall or bubble domains. Typically, the bubble domains are generated by applying a suitable bias field perpendicular to a layer of magnetic bubble domain material. The normal bubble domains that are induced in such a material exist over a narrow range of bias field values, typically about 10 Oersteds, and propagate in the direction of an applied bias field gradient. However, in garnet materials, bubble domains may be formed that exist over a range of bias field values of as much as 40 Oersteds. In addition, these unusual bubble domains, termed hard bubbles, have low mobilities and propagate at an angle to the applied bias field gradient. Because of such properties, the presence of hard bubbles may render the garnet material unsuitable for use in bubble domain circuits and devices.
Several techniques are available for suppressing the formation of hard bubble domains. A double layer technique (Type I) is described in an article by A. H. Bobeck et al, published in the Bell System Technical Journal, Vol. 51, pgs. 1431-35, July-August, 1972. In this technique, a garnet layer of low magnetic moment is interposed between a garnet bubble domain layer and a substrate. The application of a suitable bias field to form bubble domains in the bubble layer saturates the suppression layer, precluding the formation of bubble domains therein and magnetizing the entire suppression layer antiparallel to the bubble domains. As a result of the antiparallel directions of magnetization, domain walls are formed between the intermediate layer and the bubble domains. These extra domain walls, termed 180.degree. walls because of the antiparallel magnetization, apparently suppress the formation of hard bubbles by limiting the degrees of freedom available to the domain wall geometry. The usefulness of this suppression technique is limited by the propensity of the suppressed bubble layer to spontaneously generate unwanted bubbles.
Another double layer suppression technique (Type II) is described in the paper by A. H. Bobeck et al, supra. This technique utilizes a garnet bubble domain layer having a magnetization compensation temperature below room temperature. An underlying garnet layer possesses a lower moment and has a compensation temperature above room temperature. Upon application of an external bias field to form bubble domains in the bubble domain layer and to saturate the interposed film, the d-site Fe sublattices of the underlying layer and the non-bubble regions of the bubble domain layer are magnetized in antiparallel directions. This creates interfacial domain walls external to the bubble domains. That is, domain walls are created at the interface of the two layers between, but not along, the lower end of the bubble domains. The authors report that hard bubbles are eliminated by such a domain wall. However, as may be appreciated, the operability of this arrangement is limited to a narrow temperature range and may be temperature sensitive within that range.
A single-layer hard bubble suppression technique that utilizes ion implantation to form a wall or boundary in the upper surface of a magnetostrictive garnet bubble domain layer is described by R. Wolf and J. C. North in the Bell System Technical Journal, Vol 51, pgs. 1436-1440, July-August, 1972. The ion implantation is accomplished in a thin region in the upper surface of the garnet layer. The constraints exerted by the rest of the layer on the implanted region create a net moment of magnetization parallel to the surface. The magnetization of the implanted region apparently creates an extra domain wall in bubble domains induced in the uniplanted region of the layer, thereby eliminating hard bubble domains by decreasing the number of available degrees of freedom. However, the ion implantation technique is limited to garnet materials having negative magnetostriction constants of relatively large absolute values. In addition, the ion implanted region physically separates the generation and other device structures from the bubble domain layer and presumably renders bubble devices formed therefrom less efficient.
It may be thus appreciated that there exists a need for a bubble domain structure that efficiently suppresses the formation of hard bubble domains, is not temperature sensitive at normal operating temperatures, and is neither subject to difficulties such as the unwanted nucleation of bubbles nor to limitations such as the above-described magnetostriction requirements.